Thus far, techniques have been known for purifying nitrogen oxides (hereinafter referred to as “NOx”) contained in exhaust.
For example, in Patent Documents 1 and 2 and Non-patent Document 1, an exhaust purification device is shown that is provided with an oxidation catalyst and a NOx occlusion-reduction catalyst (hereinafter referred to as “LNT”) in an exhaust channel. With this exhaust purification device, NOx in the exhaust having passed through the oxidation catalyst is occluded by reacting with an alkali metal, alkaline earth metal or the like during lean operation in which the exhaust is oxygen excessive, and then the NOx thus occluded is reduced during rich operation in which the oxygen concentration of the exhaust is low. With this exhaust purification device, occlusion of NOx and reduction of NOx can be performed periodically by repeating lean operation and rich operation.
In addition, in Non-patent Document 2, for example, a method is shown in which NOx is adsorbed on a catalyst during lean operation in which the exhaust is oxygen excessive, then rich operation is performed and a state in which the oxygen concentration in the exhaust is low is periodically produced, while carbon monoxide is periodically synthesized and supplied, thereby periodically reducing the NOx adsorbed during lean operation.
More specifically, in the method shown in Non-patent Document 2, first, nitrogen monoxide and nitrogen dioxide existing in the exhaust is adsorbed to the catalyst during lean operation in which the exhaust is oxygen excessive, by way of the following formulas (1) to (3).NO→NO (adsorption)  (1)2NO+O2→2NO2  (2)NO2→NO2 (adsorption)  (3)
Next, rich operation is performed while carbon monoxide is synthesized. The carbon monoxide thereby synthesized produces hydrogen by way of the water-gas shift reaction shown in the following formula (4), in a situation where the oxygen partial pressure is low.CO+H2O→H2+CO2  (4)
Furthermore, ammonia is produced by reacting this hydrogen with carbon monoxide in a reducing atmosphere, and this ammonia is adsorbed to the catalyst by way of the following formula (5).5H2→2NO→2NH3 (adsorption)+2H2O  (5)
With the ammonia produced by carbon monoxide according to the above as the final reducing agent, NOx in the exhaust or NOx adsorbed to the catalyst is reduced by way of the following formulas (6) to (8).4NH3+4NO+O2→4N2+6H2O  (6)2NH3+NO2+NO→2N2+3H2O  (7)8NH3+6NO2→7N2+12H2O  (8)
Incidentally, NOx may be discharged without being reduced in the LNT up to when the LNT reaches a predetermined activation temperature, such as immediately after engine startup. A method has been known for quickly raising the temperature of the LNT in which the idle revolution speed of the engine is raised, and the main injection timing is retarded; however, according to this method, fuel economy may deteriorate.
Consequently, a method for raising the temperature of the LNT that makes the NOx purification rate optimum while controlling deterioration of the fuel economy is illustrated in Patent Document 3, for example.
Alternatively, in Patent Document 4, for example, an exhaust purification system is shown that is provided with an LNT in the exhaust channel, and is further provided upstream of this LNT with a fuel reformer that produces a reducing gas containing hydrogen and carbon monoxide by reforming hydrocarbon fuel. In this exhaust purification system in particular, a fuel reformer is used that produces a reducing gas such that hydrogen is more abundant than carbon monoxide by volume ratio. According to this system, it becomes possible to selectively reduce the NOx in the exhaust by adding reducing gas containing hydrogen from an upstream side of the LNT into the exhaust.
Here, as a method of producing reducing gas from hydrocarbon fuel, for example, a partial oxidation reaction using oxygen as an oxidant has been known, as shown in the following formula (9).CnHm+1/2nO2→nCO+1/2mH2  (9)
This partial oxidation reaction is an exothermic reaction using fuel and oxygen, and the reaction progresses spontaneously. As a result, once the reaction begins, hydrogen can be continuously produced without supplying heat from outside. In addition, in such a partial oxidation reaction, in a case of fuel and oxygen coming to coexist in a high temperature state, the combustion reaction as shown in the following formula (10) also progresses.CnHm+(n+1/4m)O2→nCO2+1/2mH2O  (10)
Moreover, the steam reforming reaction, which uses steam as an oxidant, shown in the following formula (11) has been known.CnHm+nH2O→nCO+(n+1/2m)H2  (11)
This steam reforming reaction is an endothermic reaction using fuel and steam, and is not a reaction that progresses spontaneously. As a result, the steam reforming reaction is an easily controlled reaction relative to the partial oxidation reaction described above.    Patent Document 1: Japanese Patent No. 2586738    Patent Document 2: Japanese Patent No. 2600492    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-2905    Patent Document 4: Japanese Patent No. 3642273    Non-patent Document 1: “Development of NOx Storage Reduction Three-way Catalyst System,” Collective Papers of Society of Automotive Engineers of Japan, Vol. 26, No. 4, October 1995    Non-patent Document 2: “A NOx Reduction System Using Ammonia Storage-Selective Catalytic Reduction in Rich and Lean Operations,” 15 Aachener Kolloquium Fahrzeug- and Motorentechnik 2006 p. 259-270